Since starch is a vegetable carbohydrate, efforts are being made to use it as a "natural plastic" in a wide variety of areas using existing methods of processing plastics. Due to their granular structure, however, natural starches must first be broken down or destructurized, before they become thermoplastically processable. Although they are biodegradable and have then the characteristics of thermoplastic plastics, the starches do not have their desired good properties. In order to achieve these properties, thermoplastic starch compositions of this type must be further improved. However, this frequently causes them to lose their full biodegradability.
EP 344 118 A2 relates to a polymer blend material comprising a melt of at least one water-containing destructurized hydrophilic polymer and at least one synthetic, essentially water-insoluble thermoplastic polymer. Examples are blends based on gelatin and cellulose derivatives, and polyethylene, polystyrene, polyvinyl ether, polyoxymethylene, and ethylene-acrylic acid copolymers which show an improvement in dimensional stability resulting from the addition of the water-insoluble polymer. However, the reference made therein to a possible biodegradability, namely the loss of this additional stability after several days, is instead something of a disadvantage for products molded from the blends, without actually saying anything regarding biodegradability itself.
EP-A 327 505 A2 describes a melt-mixed polymer blend material comprising destructurized, - but chemically unmodified-starch and at least one water-insoluble, synthetic thermoplastic polymer.
In the same way, EP-A 409 789 A2, EP-A 409 788 A2, EP-A 409 783 A2, EP-A 409 782 A2, EP-A 409 781 A2, EP-A 408 503 A2, EP-A 408 502 A2, EP-A 408 501 A2, EP-A 407 350 A2, EP-A 404 728 A2, EP-A 404 723 A2, and EP-A 404 727 A2 disclose polymer blend compositions comprising destructurized, but chemically unmodified, starch and a functionalized polymer. Each of these compositions may additionally contain a water-insoluble, thermoplastic polymer. The functionalized polymer then acts as a compatibility enhancer between the chemically unmodified starch and the additional third thermoplastic polymer.